1) Field of the Invention
The field of the invention generally relates to amplifiers and, more specifically, to an improved method to minimize distortion and power dissipation in a compact amplifier.
2) Background
Amplifiers are used for many applications including audio signal processing, video processing, communications, control systems, satellites, and so on. Based on its power consumption characteristics, an amplifier may be categorized into one or more categories including Class A, B, AB, D, G or H. A general background of different amplifier classifications may be found, for example, in R. Bortoni, et al., “Analysis, Design and Assessment of Class A, B, AB, G and H Audio Power Amplifier Output Stages Based on MATLAB® Software,” 110th Convention of Audio Engineering Society (AES), May 12-15, 2001, pp. 1-14, and in “Audio Power Amplification,” J. Audio Eng. Soc., Vol. 54, No. 4, April 2006, pp. 319-323, both of which publications are hereby incorporated as if set forth fully herein.
A Class A amplifier is known to require a relatively large amount of standing power and is inefficient, and thus undesirable in many applications that require low power dissipation. A Class B amplifier can have very low standing power dissipation but introduces distortion. A Class AB amplifier is somewhere between the two, and uses bias currents to reduce the distortion inherent in a Class B amplifier. A Class AB amplifier has a higher power dissipation than a Class B amplifier due to the bias currents.
As equipment gets smaller, amplifiers become an increasing limitation on the ability to reduce packaging size. The inevitable limited efficiency of an amplifier leads to power dissipation that must be dissipated by a heatsink in order to prevent the amplifier getting too hot; however, a large heatsink can take up an inordinate amount of packaging space. Techniques exist to improve the efficiency over that of a standard Class AB amplifier design, and hence reduce heatsink requirements, but these approaches often lead to compromises in the bandwidth, noise or distortion performance of the amplifier.
There are at least two aspects to the power dissipation of an amplifier system. The first is commonly known as idle or standing dissipation—that is, power dissipated when the amplifier is delivering no power to the load. With a conventional linear amplifier, this standing dissipation, which includes the bias current applied to the amplifier, is primarily concentrated in the driver and output stage of the amplifier and is generally required in conventional amplifier designs to minimize crossover distortion. In a high performance audio power amplifier of a nominal 100 Watt sine wave power capability into an 8 Ω load, for example, the idle current may be of the order of 100-200 milli-amps per channel. With a quiescent power supply voltage of typically +/−45 Volts, this idle current results in roughly 9-18 Watts of idle power dissipation per channel. This can be a significant problem for a stereo amplifier, but for a multi-channel amplifier it is an even larger problem, as the idle dissipation quickly becomes excessive as the number of amplifiers is increased.
For home audio amplifiers, large heatsinks can often be used to dissipate the power and keep the temperature of the power devices down, but for size-constrained applications such as automotive entertainment systems, the size and weight of the heatsink cannot be tolerated.
The idle current for an amplification system must generally be set at the time of production for optimum distortion performance and thus increases production costs. The idle current requirements can also change with temperature and age. Therefore, over time or after prolonged use, the quality of the amplifier output may deteriorate.
To add to the idle power dissipation problem, an amplifier produces extra dissipation, sometimes referred to as dynamic dissipation, when it is delivering a signal to the load. In practice, a nominal 100 Watt capability linear power amplifier may well dissipate 40 Watt worst case when delivering sine wave signals to a load. With music as the audio source for the amplifier, this figure is lower since music has a higher crest factor than sine waves, but may still approach 30 Watts per channel.
Various techniques have been employed to reduce both the idle power dissipation and the dynamic power dissipation of linear amplifiers. One technique that can be employed to reduce idle power dissipation is to decrease the output stage bias current. However, this causes crossover distortion to increase which is difficult to eradicate with conventional negative feedback around the amplifier. Also, this technique has little effect on the dynamic power dissipation.
Another approach that can reduce both idle and dynamic dissipation is to use a ‘Class G’ amplifier configuration. This ‘Class G’ nomenclature is commonly attributed to Hitachi (see “Highest Efficiency and Super Quality Audio Amplifier Using MOS Power FETs in Class G Operation,” IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3, August 1978), although the basic technique appears to have been described previously (see, e.g., U.S. Pat. No. 3,622,899). A ‘Class G’ amplifier arrangement maintains a lower voltage across the output devices under idle conditions whilst also reducing dynamic power dissipation by ensuring that the voltage across the power devices is also reduced when driving signals to the load. Thus both the idle and dynamic power dissipation is reduced. However, the switching of the output devices between the power rails often causes glitches in the output waveform that appear as distortion. These glitches have significant high frequency energy and so are difficult to correct by negative feedback. Careful design can reduce this effect but cannot eliminate it and tends to increase high frequency dynamic power dissipation.
An alternative way of reducing amplifier dissipation is to implement a switching amplifier, and specifically a so-called ‘Class D’ architecture. With this design, the linear amplifier is replaced by power switches operating at typically several hundred kilohertz for a high performance audio amplifier. The nominal efficiency of this design into a resistive load can theoretically be very high, although in practice switching losses and output filter losses significantly reduce the actual efficiency. The high switching frequency can cause significant EMI problems which then require bulky inductors to prevent coupling to power supply and output lines, as well as careful screening to avoid radiation. These additions mean that although the basic amplifier components can be small and low cost, the overall size is significantly larger and more costly due to the need for the inductive and filter components. Furthermore, the continual switching causes a significant idle current due to the dynamic switching losses and the pulse width modulation (PWM) process used to generate the switching signal leads to a poor distortion performance compared to a linear amplifier.
An example of a Class D amplifier is the model TDF8590TH amplifier available from NXP Semiconductors, a company headquartered in the Netherlands. When this amplifier is configured to provide a nominal 100 Watt sine wave power to an 8 Ω load, the idle dissipation is in excess of 4 Watts/channel. The total harmonic distortion (THD) is above 0.1% at 10 kHz at all levels above 10 Watts output and rises dramatically at higher output levels—and even these figures are an underestimate of the actual distortion due to the use of an AES17 filter to remove the effect of the residual switching frequency components on the measuring equipment. The intermodulation distortion (IMD) performance is much worse than a well-designed linear amplifier. The output inductors also generally must be large, in order that they do not saturate or introduce further distortion, and typically measure 4 to 5 cubic centimeters, which is a barrier to overall circuit and package miniaturization.
There remains a need therefore for an amplifier topology that can be readily miniaturized while providing low power dissipation. There further is a need for an amplifier that is capable of providing low idle and dynamic power dissipation levels, requires no bias setting, and no inductors for EMI or filtering. There further is a need for an amplifier that delivers very low distortion levels.